Why are there no living animals as large as the dinosaurs?
--Steve Tidwell, Orlando, Florida
Matthew Carrano, a vertebrate paleontologist at the Smithsonian Institution in Washington, D.C., responds:
There are no obvious structural reasons why terrestrial mammals could not be as large as the giant dinosaurs, so other factors must be at work. Some scientists have suggested that plants were more productive in the Mesozoic, providing enough food to sustain huge herbivores.
More likely, mammals are limited to smaller sizes because they gestate their young internally. Large mammals tend to have long gestation times—nearly two years for elephants—in part because most feed on plants that have relatively low nutritional value.
A lengthy gestation period has two important side effects: The mothers almost always have single births, and the loss of an individual baby is tremendously costly. It is especially difficult for the largest mammals to recover from population disturbances such as disease or drought, so they may be susceptible to extinction.
As egg layers, dinosaurs most likely faced no such problems. They could probably lay a large clutch several times a year if necessary and thereby recover more quickly from major disturbances.
Of course, there is one group of living animals that rival the dinosaurs in size: whales. Whales do not have to deal with gravity, so they can avoid the structural limits imposed on terrestrial mammals and dinosaurs alike. Whales are also under slightly less environmental pressure to reproduce than terrestrial mammals because their young grow quickly, nourished by the abundant, high-protein diet available in the ocean.